Most new aromatics complexes are designed to maximize the yield of benzene and C8 aromatic isomer (para-xylene, meta-xylene, ethylbenzene and ortho-xylene). Benzene is a versatile petrochemical building block used in many different products based on its derivation including ethylbenzene, cumene, and cyclohexane. In many instances, the sought C8 aromatic isomer is para-xylene as para-xylene is an important building block, which is used almost exclusively for the production of polyester fibers, resins, and films formed via terephthalic acid or dimethyl terephthalate intermediates. Accordingly, an aromatics complex may be configured in many different ways depending on the desired products, available feedstocks, and investment capital available. A wide range of options permits flexibility in varying the product slate balance of benzene and para-xylene to meet downstream processing requirements.
A prior art aromatics complex flow scheme has been disclosed by Meyers in the HANDBOOK OF PETROLEUM REFINING PROCESSES, 2d. Edition in 1997 by McGraw-Hill.
In an aromatics complex, the production of commercial-grade C8 aromatic isomers involves multiple separation steps such as fractionation, adsorptive separation and/or crystallization and reaction steps including transalkylation, isomerization, dealkylation etc. In typical aromatic complexes used to produce high purity C8 aromatic isomers, the isomer-depleted raffinate stream from the separation process, either simulated moving bed adsorption or crystallization, is sent to an isomerization process in which the remaining xylene isomers are isomerized to produce the desired isomer (near equilibrium concentration) and convert ethylbenzene (EB) to other components which can be separated via fractionation or other means. One type of xylene isomerization process also isomerizes EB to xylenes via a C8 naphthene intermediate. In a typical paraxylene complex using simulated moving bed (SMB) adsorption, the C8 naphthenes remain in the feed to the SMB unit. This is feasible due to the typical use of para-diethylbenzene as the desorbent in the SMB process. In paraxylene complexes using toluene as the SMB desorbent, the majority of the C8 napthenes must be removed from the feed to the SMB unit in order to avoid accumulation of the C8 naphthenes in the circulating toluene desorbent. This also holds true for complexes designed to produce meta-xylene which also utilizes toluene as the SMB desorbent. This is necessary because the C8 naphthene isomers have boiling points within the range of toluene (at the low end) and xylenes (at the high end). As a result, C8 isomers, such as 1,1,3-trimethyl cyclopentane which has a boiling point slightly lower than toluene, will accumulate in the toluene desorbent supply, thus making the material less suitable as a desorbent. In addition, because some of the C8 naphthene isomers have boiling points close to toluene and toluene is a byproduct produced in the xylene/EB isomerization process, it is also necessary to remove toluene from the C8 naphthene recycle stream to avoid accumulation of toluene in the isomerization feed.
Further, the feed stream separated from C8 naphthenes which is passed to the xylene separation unit may contain higher molecular weight hydrocarbons i.e. C9+ components as contaminants making the xylene separation process inefficient.
Accordingly, it is desirable to provide improved methods and apparatuses for separating C8 naphthenes from an isomerate product stream before being recycled to the xylene separation unit. Further, it is desirable to provide a cost-effective method and apparatus to solve the problem of accumulation of C8 naphthenes in the desorbent supply in a xylene separation unit. Also, it is desirable to remove toluene from the C8 naphthene recycle stream to avoid accumulation of toluene in the isomerization feed. Moreover, it is desirable to reduce the amount of C9+ alkylaromatic hydrocarbons being passed to the xylene separation unit as contaminants in an efficient and cost-effective way. Furthermore, other desirable features and characteristics of the present subject matter will become apparent from the subsequent detailed description of the subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the subject matter.